A process for the catalytic ring-opening repolymerization of cyclic dimers in nylon 6 extractable oligomers under mild conditions

Abstract

This invention discloses a method for the ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions. The method includes mixing cyclic dimer powder with a sulfonic acid catalyst and water to obtain a mixture; heating the mixture to a first reaction temperature and reacting it in an inert gas atmosphere; vacuum drying the reaction product; adding water and raising the temperature to a second reaction temperature for prepolymerization in an inert gas atmosphere; after prepolymerization, cooling to a third reaction temperature and performing devolatilization under vacuum to obtain high molecular weight recycled nylon 6. This invention effectively opens the cyclic dimer at a lower temperature using a sulfonic acid catalyst, effectively reducing catalyst dosage and mitigating equipment corrosion. The ring-opening product can undergo polymerization to achieve the recycling and reuse of extractable nylon 6 oligomers and effectively control the total content of extractable oligomers in recycled nylon 6 chips.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions. Background Technology

[0002] Nylon 6 chips, as a high-performance engineering plastic, possess outstanding advantages such as wear resistance, high strength, chemical corrosion resistance, and ease of processing. Currently, the most widely used industrial polymerization process for nylon 6 is caprolactam hydrolysis ring-opening polymerization, a reversible equilibrium reaction. The equilibrium product contains approximately 10 wt% extractable oligomers, mainly cyclic oligomers, of which caprolactam accounts for about 9 wt%, and the remaining cyclic oligomers about 1 wt%. Cyclic dimers (CD) account for about 90% of the total cyclic oligomer content. CD has a melting point of 348℃, far exceeding that of nylon 6, and is stable and does not readily undergo ring-opening. If directly recycled or repolymerized, the cyclic dimer content in the product will be excessively high, severely reducing the quality of the chips. Furthermore, CD easily precipitates during production, severely clogging pipes and easily causing filament breakage during spinning, reducing the mechanical properties and appearance of the final product. This also prevents large-scale melt spinning of nylon 6 at present. To address this, the industry primarily removes residual extractable oligomers (especially cyclic dimers CD) from nylon 6 chips through hot water extraction, and focuses on developing mild and efficient catalytic cracking technologies for oligomers to recover caprolactam or reactive linear oligomers from the extractable oligomers, thereby achieving resource recycling and green production.

[0003] Currently, there are two main methods for recycling nylon 6 cyclic oligomers. One method involves high-temperature pyrolysis of the cyclic dimers. The nylon 6 extract concentrate is pyrolyzed at temperatures above 270°C to convert the cyclic dimers into caprolactam and linear oligomers, which are then added to the fresh caprolactam for reuse. While this high-temperature pyrolysis method effectively reduces the cyclic dimer content in the concentrate to meet reuse standards, the stringent temperature requirements make large-scale industrial application difficult. The other method is catalytic depolymerization, using catalysts such as phosphoric acid to catalyze the depolymerization of cyclic oligomers. However, this process typically involves temperatures above 230°C, and the higher temperatures exacerbate acid corrosion of metals, placing extremely high demands on the equipment.

[0004] Chinese patent CN119701825A discloses a catalytic depolymerization method for extractable nylon 6 oligomers. The method comprises the following steps: mixing and heating a concentrated extractable nylon 6 oligomer solution with a depolymerization catalyst and then ultrasonically treating it to obtain a premix; performing a staged gradient depolymerization of the premix under an inert gas atmosphere to obtain a depolymerization product; and then purifying the depolymerization product to obtain caprolactam. The specific process of the staged gradient depolymerization includes: the reaction pressure and temperature within the depolymerization reaction system undergoing a staged gradient change with reaction time until the catalytic depolymerization reaction of the nylon 6 extractable oligomers is completed. This depolymerization method can achieve a cyclic dimer cracking rate of over 90%, significantly improving the depolymerization efficiency of cyclic dimers. However, in the staged depolymerization process, the temperature is consistently between 200℃ and 300℃, and a relatively high amount of solid acid catalyst with a complex preparation method is required.

[0005] Chinese patent CN119708464A discloses a method for producing nylon 6 polymers with extractable oligomers that can be fully recycled. The method involves the following steps: polymerizing the reactants, then cooling and pelletizing them to obtain crude nylon 6 chips. Extracting the crude nylon 6 chips yields wet nylon 6 chips and an oligomer extract. This extract is then concentrated in an evaporation and concentration section to obtain a concentrated oligomer extract with a water content of 50wt%–90wt%. This concentrate is then subjected to catalytic cracking at 230℃–350℃ and 1MPa–8MPa pressure to obtain cracking products, all of which are returned to the nylon 6 polymerization section for recycling. This cracking process uses a phosphoric acid catalyst, achieving a depolymerization rate of over 90% for cyclic dimers and a caprolactam conversion rate in the extractable oligomers. However, the high cracking temperature exacerbates the corrosiveness of phosphoric acid, placing high demands on the equipment.

[0006] Therefore, this invention relates to a method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions. This method enables the recovery and utilization of cyclic dimers in extractable nylon 6 oligomers under mild conditions. Compared with the traditional phosphoric acid cracking process, the reaction temperature is significantly reduced, thereby effectively mitigating the corrosion of equipment by acid. The method also involves low catalyst dosage, fewer impurities, simple operation, and effective control of the total content of extractable oligomers in regenerated nylon 6 chips. This has significant implications for the production process of nylon 6 chips. Summary of the Invention

[0007] The objective of this invention is to provide a method for the ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions. Using a sulfonic acid catalyst, the relatively stable cyclic dimer CD can be effectively ring-opened at a lower temperature, followed by polymerization to generate high-molecular-weight regenerated nylon 6. This method achieves the recovery and reuse of cyclic dimers CD from nylon 6 extractable oligomers and effectively controls the total content of extractable oligomers in the regenerated nylon 6 chips. The reaction temperature range of this method is 120℃~180℃, far lower than the required temperature of existing technologies, enabling efficient ring-opening of cyclic dimers under mild conditions.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by this invention is: a method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions, comprising the following steps: (1) Ingredients: Nylon 6 extractable oligomers are separated to obtain cyclic dimer powder, which is then mixed with sulfonic acid catalyst and water to obtain a mixture; (2) Reaction: The mixture obtained in step (1) is heated to the first reaction temperature and the reaction is carried out in an inert gas atmosphere; (3) Drying: The product after the reaction in step (2) is vacuum dried to ensure the accuracy of the amount of water used in step (4); (4) Prepolymerization: Add water to the product after vacuum drying in step (3), heat to the second reaction temperature, and carry out the reaction in an inert gas atmosphere; (5) Polycondensation: After prepolymerization, the temperature is lowered to the third reaction temperature and devolatilization is carried out under vacuum conditions. This can promote polycondensation while inhibiting the regeneration of cyclic oligomers caused by high temperature during the devolatilization process, thus obtaining high molecular weight regenerated nylon 6.

[0009] Preferably, the extractable oligomer includes one or more combinations of caprolactam CL, cyclic dimer CD, cyclic trimer CT, cyclic tetramer to cyclic nonamerm; The general structural formula of the extractable oligomer is: n=1~9; The structural formula of the cyclic dimer is as follows: .

[0010] Preferably, the extractable oligomer composition is: caprolactam CL 90wt%~99wt%, cyclic dimer CD 0.9wt%~5wt%, and the total content of the remaining oligomers 0.1wt%~5wt%.

[0011] Preferably, in step (1), the method of separating the extractable oligomers of nylon 6 to obtain cyclic dimer powder is based on the principle that different oligomers have different solubilities and sublimation characteristics at room temperature. The extract produced by the hot water extraction process in the continuous production of nylon 6 is separated and purified by evaporation, concentration and drying to obtain CD with a purity of more than 98%.

[0012] Preferably, the specific method of step (1) is as follows: in a high-pressure reactor, the cyclic dimer powder is mixed with sulfonic acid catalyst and water to obtain a mixture, and the reactor is sealed.

[0013] Preferably, in step (1), the sulfonic acid catalyst is one or more of the following: dodecylbenzenesulfonic acid, p-toluenesulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, β-aminoethanesulfonic acid, and dinonylnaphthalenedisulfonic acid.

[0014] Preferably, in step (1), the weight ratio of cyclic dimer CD powder to water is 1:0.01 to 1:30; Preferably, in step (1), the amount of sulfonic acid catalyst is 0.01wt% to 5wt% of the cyclic dimer CD powder.

[0015] Preferably, in step (1), the weight ratio of the cyclic dimer powder to water is 5 to 15.

[0016] Preferably, the specific method of step (2) is as follows: replace the air in the reactor several times with an inert gas. Then raise the temperature to the first reaction temperature. At this time, the system pressure will rise to the corresponding saturated vapor pressure, i.e., the reaction pressure, due to the volatilization of water vapor and reaction products. Stir the reaction for a certain period of time under these conditions.

[0017] Preferably, in step (2), the first reaction temperature is 140℃~160℃, the reaction pressure is 0.1Mpa~2Mpa, the reaction time is 2~8h, and the reaction stirring speed is 50~500r / min; after the reaction, the product is divided into liquid and solid phases; the reaction conversion rate of the cyclic dimer is 50%~90%.

[0018] Preferably, in step (2), the first reaction temperature is 140℃~160℃ and the reaction time is 2~8h.

[0019] Preferably, the specific method of step (3) is as follows: after the reaction in step (2) is completed, cool to room temperature, depressurize, and the reaction product is divided into liquid and solid phases. The product is then vacuum dried at low temperature to completely remove water for later use.

[0020] Preferably, in step (4), the second reaction temperature is 240℃~260℃, the reaction pressure is 0.2Mpa~2Mpa, and the reaction time is 4h; The number average molecular weight of the prepolymer is 2000 g / mol to 12000 g / mol; the total content of extractable oligomers is 2 wt% to 20 wt%, of which the content of cyclic dimers is 0.1 wt% to 1 wt%.

[0021] Preferably, in step (5), the third reaction temperature is 240℃, the reaction pressure is 100Pa, and the reaction time is 2h; The number average molecular weight of the condensation product is 15,000 g / mol to 30,000 g / mol; the total content of extractable oligomers is 0.5 wt% to 2 wt%, of which the content of cyclic dimers is 0.02 wt% to 0.1 wt%.

[0022] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: 1. This invention utilizes a sulfonic acid catalyst to effectively open the ring-shaped dimers in extractable oligomers at a lower temperature, generating caprolactam, aminocaproic acid, or linear oligomers. The products can undergo polymerization to generate high molecular weight regenerated nylon 6, realizing the recycling and reuse of cyclic dimers in nylon 6 extractable oligomers, and effectively controlling the total content of extractable oligomers in regenerated nylon 6 chips.

[0023] 2. The present invention can control the temperature of the ring-opening reaction at 120℃~180℃, which is much lower than the required temperature of the prior art. Compared with high-temperature reaction, it effectively reduces the energy consumption of the process.

[0024] 3. Compared with similar technologies, the reaction of this invention requires less catalyst and produces lower impurities in the final product. Compared with the traditional phosphoric acid cracking process, the reaction temperature is significantly reduced, thus effectively mitigating the corrosion of equipment by the acid; moreover, the sulfonic acid used is significantly less corrosive to metals than phosphoric acid at the same temperature, and the reaction catalyst requires less catalyst, produces fewer impurities, and the operation process is simple. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, some of the drawings in the following description are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is the curve showing the change in CD conversion rate with reaction time in Example 3; Figure 2 This is the time-of-flight mass spectrum of the product at 120°C in Example 7; Figure 3 This is the time-of-flight mass spectrum of the product at 180°C in Example 7; Figure 4 This is a graph showing the change in CD conversion rate over reaction time obtained by reacting with sulfonic acid catalyst in Example 3 and without catalyst in Comparative Example 1. Figure 5 This is a graph showing the change in CD conversion rate over reaction time obtained by reacting with sulfonic acid catalyst in Example 4 and phosphoric acid catalyst in Comparative Example 2, respectively. Figure 6 This is the time-of-flight mass spectrum of the product when trifluoromethanesulfonic acid is used as a catalyst; Figure 7 This is the time-of-flight mass spectrum of the product when p-toluenesulfonic acid is used as a catalyst. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Unless otherwise specified, conditions not specifically described in the embodiments of this invention shall be performed under conventional conditions or conditions recommended by the manufacturer. Raw materials and reagents used without specified manufacturers are all commercially available products. All materials involved in the following embodiments are commercially available.

[0029] Example 1 This embodiment relates to a method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions, including the following steps: (1) Ingredients: The extractable oligomer of nylon 6 is separated to obtain cyclic dimer powder. In a high-pressure reactor, the cyclic dimer powder is mixed with sulfonic acid catalyst and water to obtain a mixture. The reactor is then sealed. (2) Reaction: The mixture obtained in step (1) is heated to the first reaction temperature and the reaction is carried out in an inert gas atmosphere; (3) Drying: The product after the reaction in step (2) is dried under vacuum; (4) Prepolymerization: Add water to the product after vacuum drying in step (3), heat to the second reaction temperature, and carry out the reaction in an inert gas atmosphere; (5) Polycondensation: After prepolymerization is completed, the temperature is lowered to the third reaction temperature and devolatilization is carried out under vacuum conditions.

[0030] The preparation of the cyclic dimer powder raw material in step (1) includes: evaporating, concentrating and drying the extract produced by the hot water extraction process in the continuous production of nylon 6 to obtain an extractable oligomer powder composed of caprolactam CL, cyclic dimer CD and other oligomers. The extractable oligomer powder was separated to obtain cyclic dimer CD powder.

[0031] The specific method for step (2) is as follows: replace the air in the reactor several times with an inert gas, such as nitrogen. Then raise the temperature to the first reaction temperature of 140℃. At this time, the system pressure will rise to the corresponding saturated vapor pressure due to the volatilization of water vapor and reaction products, that is, the reaction pressure is 0.45MPa. Under these conditions, stir the reaction for 6 hours at a stirring speed of 200r / min.

[0032] The specific method of step (3) is as follows: After the reaction in step (2) is completed, cool to room temperature, release pressure, and the reaction product is divided into liquid and solid phases. Dry the product under vacuum at low temperature to completely remove water for later use.

[0033] The specific method of step (4) is as follows: the dried product from step (3) is added to the polymerization reactor, heated to the second reaction temperature of 260°C, the reaction pressure is 1 MPa, and the reaction is carried out in an inert gas atmosphere for 4 hours.

[0034] The specific method of step (5) is as follows: cool down to the third reaction temperature of 240℃, carry out the devolatilization and polycondensation reaction at 100Pa for 2 hours to obtain high molecular weight recycled nylon 6.

[0035] Example 2 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0036] The purpose of this embodiment is to verify the effect of different types of sulfonic acid catalysts on the ring-opening of cyclic dimers. The differences in type, acidity, and substituents of each sulfonic acid catalyst are shown in Table 1.

[0037] Table 1

[0038] In this regard, this embodiment includes: Add 1g of cyclic dimer powder to each reactor, and add 0.01g of each of the different types of sulfonic acid catalysts to each reactor, along with 10g of deionized water. After purging the air with nitrogen three times, seal the reactors. Start stirring and set the speed to 200 rpm. Heat to 140℃ and react at this temperature for 6 hours. The reaction pressure is 0.45 MPa. After the reaction is complete, cool and depressurize. A colorless or pale yellow liquid and a small amount of white solid are obtained. After the product is completely dried, extract with methanol and collect the extract for later use. Analyze the sample using high-performance liquid chromatography (HPLC). The experimental results are shown in Table 2 below.

[0039] Table 2

[0040] Tables 1 and 2 clearly show that when different sulfonic acids are used as catalysts, the CD conversion rates from highest to lowest are: trifluoromethanesulfonic acid, p-toluenesulfonic acid, methanesulfonic acid, and benzenesulfonic acid. This indicates that the catalyst acidity is directly proportional to the CD conversion rate; under the same acidity, the presence of methyl substituents enhances the catalytic effect. The time-of-flight mass spectrum of the product when trifluoromethanesulfonic acid is used as a catalyst is shown below. Figure 6 As shown, there are many byproducts in the product. The time-of-flight mass spectrum of the product when p-toluenesulfonic acid is used as a catalyst is shown below. Figure 7 As shown, the product is relatively pure, indicating that excessively strong catalyst acidity will lead to an aggravation of side reactions.

[0041] Therefore, the preferred sulfonic acid catalyst is p-toluenesulfonic acid.

[0042] Example 3 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0043] The purpose of this embodiment is to verify the effect of sulfonic acid catalyst on CD ring opening within a temperature range of 120℃~180℃.

[0044] In this regard, this embodiment includes: 1 g of cyclic dimer powder was placed in a 40 mL high-pressure reactor, along with 10 g of deionized water and 0.01 g of p-toluenesulfonic acid. The air was purged with nitrogen three times, and the reactor was then sealed. Stirring was started at 200 rpm. The temperature was raised to 140 °C, and the reaction was carried out at this temperature for 2 h, 4 h, 6 h, 8 h, and 12 h, respectively. The reaction pressure was 0.45 MPa. After the reaction was completed, the mixture was cooled and the pressure was released. A colorless or pale yellow liquid and a small amount of white solid were obtained. After the product was completely dried, it was extracted with methanol, and the extract was collected for later use. The sample was analyzed by high-performance liquid chromatography (HPLC). The experimental results are shown in Table 3 below.

[0045] Table 3

[0046] From Table 3 above and Figure 1 It can be clearly seen that the conversion rate of CD can reach 65.5%, indicating that the ring-opening reaction of CD can occur under sulfonic acid catalysis at this temperature.

[0047] Example 4 This embodiment is based on the above embodiment 3, and the similarities with the above embodiment 3 will not be repeated.

[0048] The purpose of this embodiment is to verify the optimal temperature for catalyzing the ring-opening of CD using a sulfonic acid catalyst.

[0049] In this regard, this embodiment includes: The reaction temperatures used in this embodiment were 120℃, 140℃, 150℃, 160℃, and 180℃, respectively; the reaction time was 8 hours; the remaining steps were the same as in Example 3; and the results obtained are shown in Table 4.

[0050] Table 4

[0051] As can be clearly seen from Table 4 above, in the range of 120℃ to 180℃, under the condition of 1wt% catalyst dosage, the equilibrium conversion of CD first increases and then decreases, with 150℃ being the optimal temperature and the conversion rate being 68.8%.

[0052] Example 5 This embodiment is based on the above embodiment 3, and the similarities with the above embodiment 3 will not be repeated.

[0053] The purpose of this embodiment is to verify the optimal water content for catalyzing the ring-opening of CD using a sulfonic acid catalyst.

[0054] In this regard, this embodiment includes: The mass ratio of cyclic dimer CD powder to water used in this embodiment is 1:0.01, 1:0.1, 1:0.5, 1:1, 1:2, 1:5, 1:10, 1:20, and 1:30; the reaction temperature is 150℃ and the reaction time is 8h; the remaining steps are the same as in Example 3, and the results are shown in Table 5.

[0055] Table 5

[0056] As can be clearly seen from Table 5 above, under the conditions of reaction temperature of 150℃, catalyst dosage of 1wt%, and reaction time of 8h, the higher the water content, the higher the equilibrium conversion rate. However, when the mass ratio of CD to water exceeds 1:10, the increase in conversion rate is not significant. From the perspective of economy and efficiency, a water content of 1:10 is preferred.

[0057] Example 6 This embodiment is based on the above embodiment 3, and the similarities with the above embodiment 3 will not be repeated.

[0058] The purpose of this embodiment is to verify the optimal catalyst dosage for catalyzing the ring-opening of CD using a sulfonic acid catalyst.

[0059] In this regard, this embodiment includes: In this embodiment, the amount of sulfonic acid catalyst used was 0.1 wt%, 1 wt%, 2.5 wt%, and 5 wt% of the cyclic dimer powder; the reaction temperature was 150 °C; the remaining steps were the same as in Example 3, and the results obtained are shown in Table 6.

[0060] Table 6

[0061] As can be clearly seen from Table 6 above, when the catalyst dosage is 2.5 wt%, the reaction equilibrium time is effectively shortened, while at higher dosages, the reaction rate does not increase further. Therefore, the optimal catalyst dosage is 2.5 wt%.

[0062] Example 7

[0063] This embodiment is based on the above embodiment 4, and the similarities with the above embodiment 4 will not be repeated.

[0064] The purpose of this embodiment is to analyze the feasibility of repolymerization of the product after CD ring-opening catalyzed by sulfonic acid catalyst.

[0065] This embodiment includes: mass spectrometry analysis of the products from Example 4 at 120℃ and 180℃. Figure 2 This is the time-of-flight mass spectrum of the product at 120℃. Figure 3 This is the time-of-flight mass spectrum of the product at 180℃.

[0066] Result: From Figure 2 It can be seen that the main components of the product at 120℃ are caprolactam (114), aminocaproic acid (131), CD (227), and linear dimer (245). From... Figure 3 It can be seen that the main components of the product at 180℃ are CD (227), linear dimer (245), cyclic trimer (339), linear trimer (358), cyclic tetramer (452), linear tetramer (471), linear pentamer (584), and linear hexamer (697). Based on the above results, it is predicted that the product of CD ring-opening catalyzed by sulfonic acid catalyst can undergo further polycondensation reaction with increasing temperature to form polymers with higher molecular weight, proving the feasibility of further polymerization of the product of CD ring-opening.

[0067] Example 8 This embodiment is based on the above embodiment 3, and the similarities with the above embodiment 3 will not be repeated.

[0068] The purpose of this embodiment is to repolymerize the product after CD ring opening and analyze its molecular weight and extractable oligomer content.

[0069] This embodiment includes: Under optimal conditions: reaction temperature 150℃, CD to water mass ratio 1:10, reaction time 6h, and complete drying of the product. The completely dried product was placed in a sealed reactor with 2wt% water, heated to 260℃, and prepolymerized for 4h under an inert gas atmosphere at a prepolymerization pressure of 1MPa to obtain nylon 6 prepolymer. After prepolymerization, the temperature was lowered to 240℃, and vacuum devolatilization was performed for 2h to obtain the condensation product.

[0070] The obtained nylon 6 prepolymer was tested, and its number-average molecular weight (Mn) was 10882 g / mol, with a total extractable oligomer content of 7.24 wt%, including 5.88 wt% CL and 0.24 wt% CD. The obtained condensation product was tested, and its Mn was 24251 g / mol, with a total extractable oligomer content of 0.92 wt%, including 0.23 wt% CL and 0.05% CD. The extractable oligomer content meets the standards for high-quality nylon 6 chips.

[0071] Example 9 This embodiment is based on the catalytic ring-opening step in Example 3 above, and the similarities with Example 3 above will not be repeated.

[0072] The purpose of this embodiment is to investigate the effect of different prepolymerization temperatures, i.e., the second reaction temperature, on the molecular weight and extractable oligomer content of the nylon 6 product obtained after the repolymerization of the cyclic dimer ring-opening product, so as to verify the process's control effect on oligomer content.

[0073] This embodiment includes: The preferred ring-opening conditions determined in Examples 2-6 were used: reaction temperature 150°C, mass ratio of CD to water 1:10, amount of p-toluenesulfonic acid 2.5 wt%, and reaction time 6 h. After the reaction was completed, the product was completely dried to obtain the ring-opened product.

[0074] Equal amounts of the dried ring-opening product were placed in a sealed polymerization reactor, and 2 wt% (relative to the mass of the ring-opening product) of water was added to each reactor as a prepolymerization initiator. The reactor was then sealed after the air inside was replaced with inert nitrogen gas.

[0075] Five polymerization reactors were heated to different secondary reaction temperatures: 220℃, 230℃, 240℃, 250℃, and 260℃. The prepolymerization reaction pressure was 1 MPa, and the prepolymerization reaction was carried out for 4 hours at their respective temperatures and in an inert gas atmosphere.

[0076] After prepolymerization, all five systems were cooled to the same third reaction temperature of 240°C and subjected to devolatilization polymerization at 100 Pa for 2 hours to obtain the final high molecular weight regenerated nylon 6 product.

[0077] The three groups of nylon 6 condensation products were tested, and the results are shown in Table 7 below: Table 7

[0078] As can be clearly seen from Table 7 above, within the prepolymerization temperature range of 220℃ to 260℃, the number-average molecular weight (Mn) of the final product increases significantly with increasing prepolymerization temperature, rising from 18,500 g / mol to 24,250 g / mol. Simultaneously, the total content of extractable oligomers and the content of recalcitrant cyclic dimers in the product both show a decreasing trend. Particularly after prepolymerization at 260℃, the total content of extractable oligomers in the product can be controlled below 1%, and the content of cyclic dimers is only 0.05 wt%.

[0079] This embodiment demonstrates that, within the prepolymerization temperature range of 220℃ to 260℃ described in this invention, increasing the prepolymerization temperature can more effectively suppress the regeneration of extractable oligomers, especially cyclic dimers, while promoting the polymerization reaction and increasing the molecular weight of the product. This achieves good control over the content of extractable oligomers in regenerated nylon 6 chips. This proves that the method of this invention can not only achieve ring-opening of CD under mild conditions, but its subsequent repolymerization process can also effectively ensure the quality of the final product.

[0080] Example 10 The purpose of this embodiment is to verify that, under the mild reaction conditions described above, the sulfonic acid catalyst selected in this invention has significantly reduced corrosivity to the reaction equipment compared to the phosphoric acid catalyst mentioned in the background art, thereby highlighting the advantages of this technology in terms of equipment requirements.

[0081] This embodiment includes: Comparison settings: Catalyst system A (in this invention): Prepare the same reaction mixture as in Example 3: 1 g cyclic dimer powder, 10 g water, 0.01 g p-toluenesulfonic acid.

[0082] Catalyst system B (comparative example): Preparation of comparative mixture: 1g cyclic dimer powder, 10g water, 0.01g phosphoric acid (85wt% aqueous solution).

[0083] Blank control group C: 1g cyclic dimer powder, 10g water, without any catalyst added.

[0084] Corrosion evaluation methods: Prepare four metal test pieces of 304 stainless steel, clean, dry and weigh them accurately (initial mass M0).

[0085] Three metal samples were immersed in the mixtures A, B, and C respectively to obtain three reaction systems.

[0086] The three reaction systems described above were placed in a high-pressure reactor and reacted at 150°C under an inert gas atmosphere for 8 hours to simulate the core ring-opening reaction conditions of this invention. Catalyst system B was also taken, a metal sample was immersed in it, and placed in a high-pressure reactor, reacting at 250°C under an inert gas atmosphere for 8 hours to simulate the ring-opening reaction conditions of the phosphoric acid catalyst mentioned in the background art.

[0087] After the reaction is complete, the sample is cooled, removed, washed with deionized water, dried, and weighed precisely (mass M1 after reaction).

[0088] Calculate the mass loss rate of each test piece: Mass loss rate = [(M0-M1) / M0] × 100%.

[0089] Meanwhile, the surface of the test piece was observed macroscopically or microscopically, and the corrosion morphology was recorded. The results are shown in Table 8 below.

[0090] Table 8

[0091] Under the same mild reaction conditions of 150℃, the corrosiveness of the phosphoric acid catalyst to the stainless steel sample (mass loss rate of approximately 0.25%) was significantly higher than that of the p-toluenesulfonic acid catalyst (mass loss rate of approximately 0.05%), with the former being about five times greater than the latter. Corrosion in the blank control group was negligible, indicating that corrosion under these conditions was primarily caused by the catalyst. However, in the milder reaction range, the corrosiveness of sulfonic acid (mass loss rate of approximately 0.05%) was far lower than that of phosphoric acid under the higher reaction range (mass loss rate of approximately 0.40%).

[0092] This embodiment demonstrates that the sulfonic acid catalyst used in this invention is far less corrosive to equipment than the phosphoric acid catalyst commonly used in the prior art.

[0093] Comparative Example 1 This comparative example is based on Example 3 above, and the similarities with Example 3 above will not be repeated.

[0094] This comparative example does not use sulfonic acid catalyst, but only water ring-opening CD. The remaining steps are the same as in Example 3. The results are shown in Table 9 below.

[0095] Table 9

[0096] From Table 9, Table 3 and above Figure 4 It is evident that the CD conversion rate increased to 5.4 times that without the catalyst after using the sulfonic acid catalyst, demonstrating a good catalytic effect.

[0097] Comparative Example 2 This comparative example uses 85 wt% phosphoric acid as a catalyst and is compared with Example 4.

[0098] This comparative example includes: 1 g of cyclic dimer powder was placed in a 40 mL high-pressure reactor, along with 10 g of deionized water and 0.01 g of phosphoric acid catalyst. The air was purged with nitrogen three times, and the reactor was then sealed. Stirring was started at 200 r / min. The temperature was raised to 150 °C, and the reaction pressure was controlled at 0.45 MPa. The reaction was carried out at this temperature and pressure for 8 hours. The product obtained in this comparative example was tested and compared with the product generated in Example 4 at 150 °C. The results are as follows: Figure 5 As shown.

[0099] from Figure 5 It can be clearly seen that the conversion rate of CD changes with reaction time under the two conditions. Under the temperature conditions, the sulfonic acid catalyst used in Example 4 has a higher catalytic effect than the phosphoric acid catalyst, with the former having a catalytic effect 1.79 times that of the latter.

[0100] The product testing and characterization methods used in the embodiments and comparative examples of the present invention are as follows: (1) Method for testing molecular weight: Instrument: Waters ACQUITY APC Chromatographic columns: ACQUITY APC XT 45 (1.7 μm, 45 Å) + ACQUITY APC XT 200 (2.5 μm, 200 Å) + ACQUITY APC XT 450 (2.5 μm, 450 Å) Injection volume: 10 μL Flow rate: 0.4 mL / min Column temperature: 45℃ Mobile phase: hexafluoroisopropanol (containing 0.5 wt% sodium trifluoroacetate) Wash-off time: 20 min Standard: Polymethyl methacrylate Test method: Dissolve an appropriate amount of sample in the mobile phase solvent, allow it to stand in the dark until fully dissolved, filter and inject the sample into the instrument. Use standards to generate a standard curve and perform molecular weight analysis.

[0101] (2) Test method for the conversion rate of cyclic dimer CD: Instrument: Waters 2489 Column: Agilent C18 (250×4.6mm) Injection volume: 20 μL Flow rate: 1.0 mL / min Wavelength: 220nm Column temperature: 40℃ Mobile phase: The ratio of phase A to phase B varies in a gradient. Phase A is a mixture of phosphate buffer (pH=2.5) and methanol, and phase B is pure methanol.

[0102] Test method: Take an appropriate amount of sample, extract with methanol using Soxhlet extraction for 12 hours, dilute the extract to a fixed volume, and inject it into the instrument for analysis. Use CD standard to construct an external standard curve for quantitative analysis.

[0103] (3) High-resolution time-of-flight mass spectrometry analysis: Instrument: Waters Xevo G2 TOF MS Ion source: ESI+ Carrier gas: Helium Desolvation gas temperature: 400℃ Injection volume: 1 μL m / 2 range: 20-1000000Da Resolution: >2 Test method: Dilute the above methanol extract 20 times, inject it into the instrument, and analyze it.

[0104] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions, characterized in that, Includes the following steps: (1) Ingredients: Nylon 6 extractable oligomers are separated to obtain cyclic dimer powder, which is then mixed with sulfonic acid catalyst and water to obtain a mixture; (2) Reaction: The mixture obtained in step (1) is heated to the first reaction temperature and the reaction is carried out in an inert gas atmosphere; (3) Drying: The product after the reaction in step (2) is dried under vacuum; (4) Prepolymerization: Add water to the product after vacuum drying in step (3), heat to the second reaction temperature, and carry out the reaction in an inert gas atmosphere; (5) Polycondensation: After prepolymerization, the temperature is lowered to the third reaction temperature and devolatilization is carried out under vacuum to obtain high molecular weight recycled nylon 6; In step (1): the cyclic dimer powder is obtained by evaporating, concentrating, drying, separating and purifying the extract from the hot water extraction process of the continuous production of nylon 6 in the industrial plant; In step (1), the method of separating the extractable oligomers of nylon 6 to obtain cyclic dimer powder is to separate and purify the powder obtained by the hot water extraction process in the continuous production of nylon 6 through water washing and negative pressure sublimation, after evaporation, concentration and drying, so as to obtain cyclic dimers with a purity of more than 98%. In step (1), the sulfonic acid catalyst is one or more of p-toluenesulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, and benzenesulfonic acid; In step (1), the amount of sulfonic acid catalyst used is 0.01wt%~5wt% of the cyclic dimer powder; In step (2), the first reaction temperature is 120℃~180℃, the reaction pressure is 0.1MPa~2MPa, the reaction time is 1~12h, and the reaction stirring speed is 50~500r / min; In step (4), the second reaction temperature is 220℃~260℃, the reaction pressure is 0.2MPa~2MPa, and the reaction time is 4h.

2. The method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions as described in claim 1, characterized in that, The extractable oligomers include one or more combinations of caprolactam, cyclic dimers, cyclic trimers, cyclic tetramers to cyclic nonamerms; The general structural formula of the extractable oligomer is: ，n=1~9； The structural formula of the cyclic dimer is as follows: 。 3. The method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions as described in claim 1, characterized in that... In step (1), the weight ratio of the cyclic dimer powder to water is 1:0.01 to 1:

30.

4. The method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions as described in claim 1, characterized in that, In step (2), the product is divided into liquid and solid phases after the reaction is completed; the reaction conversion rate of the cyclic dimer is 40%~90%.

5. The method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions as described in claim 1, characterized in that, In step (4), the number average molecular weight of the prepolymer is 2000 g / mol to 12000 g / mol; the total content of extractable oligomers is 2 wt% to 20 wt%, of which the content of cyclic dimers is 0.1 wt% to 1 wt%.

6. The method for catalytic ring-opening and repolymerization of cyclic dimers in extractable nylon 6 oligomers under mild conditions as described in claim 1, characterized in that, In step (5), the third reaction temperature is 240℃, the reaction pressure is 100Pa, and the reaction time is 2h; The number average molecular weight of the condensation product is 10,000 g / mol to 30,000 g / mol; the total content of extractable oligomers is 0.1 wt% to 2 wt%, of which the content of cyclic dimers is 0.01 wt% to 0.3 wt%.

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